1. Field of the Invention
This invention relates to the separation of materials, particularly waste plastic materials, such separation being particularly desirable for the purpose of recycling and reusing the materials. The invention may also be used for separating other electrically insulating materials from each other, or from conductive materials such as metals, as will be explained later herein.
Waste plastic is typically derived from either garbage or from production waste during the production of plastic consumer goods, and there is a need to separate different types of plastic. Electrostatic separation, i.e. separating plastics according to their electrostatic charges, is known as one means of accomplishing this desired separation. The waste plastic is first chopped or shredded into relatively small particles, and then separated electrostatically after charging by suitable means.
Electrostatic charges build up on electrically insulating materials (including most plastics) when different such materials, having different work functions, are brought into contact. Assuming that the work function of material A is smaller than that of material B, the direct physical contact of material A on material B results in an electron transfer from A to B. The materials become tribocharged. Material A becomes positively charged, and material B, which gains the electrons, becomes negatively charged.
Tribocharging occurs only when the work functions of the two materials which are in contact are different. In some rare cases, depending on the crystal structure of the materials, one may encounter a slight difference in the work function at one point of the crystal of the material as compared to another point on the surface of the same crystal. In such a case, a very small triboelectrification may occur even between the same materials.
The different rates at which plastic materials pick up charges when contacted in this manner can be used as the basis for separating material A from material B by means of electric fields when the two materials are present in a mixture. The invention is aimed at improving the extent to which the magnitude of the tribocharge can be built up in the materials, and hence at improving the efficiency of separation.
Triboelectrification may be used only between electrically insulating materials, or between metals and electrically insulating materials. Triboelectrification may not be used to separate electrically conductive materials such as two metals. For example, if one were to contact copper and nickel, a charge transfer would occur, since the work function of the nickel is considerably higher than that of copper. However, if one then tried to separate the particles, the charge would immediately flow back to where it came from, and the net result would be that both metallic particles would be neutral after separation.
2. Description of the Prior Art
It has previously been proposed to impart electrostatic charge to particles of plastic by triboelectrification techniques involving fluidized bed technology. In such a method, a large body of the particles is caused to be in constant heaving motion, whereby the particles are constantly rubbing each other, and rubbing the bed in which they are contained. Such a method is effective in triboelectrifying the particles, but is expensive in view of the amount of energy expended in fluidizing the mass of particles, e.g. via large flows of air.
It is an aim of the invention to provide an alternative means of triboelectrification, which will operate highly effectively and with less capital cost and expenditure of energy than in fluidized bed apparatus.
It is a further aim of the invention to provide an apparatus which is versatile and simple to operate, and is compatible with other phases of electrostatic separation systems, and which can be easily adjusted to suit a variety of operating requirements.